Crystalline polypropylene is widely known in a variety of molding fields because it is excellent in mechanical property, chemical resistance, etc. However, when a propylene homopolymer is used as the crystalline polypropylene, stiffness is high, but impact resistance is insufficient. Thus, an improvement of the impact resistance has been carried out by a method of adding an elastomer such as ethylene-propylene rubbers to a propylene homopolymer or a method in which after homopolymerization of propylene, ethylene and propylene are subsequently copolymerized to produce a so-called block copolymer.
Although according to these methods, improvements of the physical properties are realized to considerable extent, a more improvement of the stiffness-impact resistance balance is desired. Also, in addition to these properties, an improvement for enhancing the heat resistance is eagerly desired.
On the other hand, it is known that isotactic polypropylene is obtained by polymerization of propylene using a metallocene based catalyst different from conventional Ziegler type catalyst systems. Also, it is known that after homopolymerization of propylene, ethylene and propylene are copolymerized using a similar catalyst, to produce a so-called block copolymer. These examples are described in, for example, JP-A-4-337308, JP-A-6-287257, JP-A-5-202152, JP-A-6-206921, and JP-A-10-219047.
Also, examples of propylene-ethylene block copolymers having high stiffness and impact resistance are described in JP-A-11-228648, JP-A-11-240929, JP-A-11-349649, and JP-A-11-349650.
According to the foregoing inventions, though the stiffness and impact resistance are more improved, not only still more improvements of the stiffness and impact resistance but also improvements in the points of heat resistance, etc. are necessary. Also, it is necessary to improve poor moldability caused by a narrow molecular weight distribution that is a general property of polymers produced using a metallocene based catalyst. In general, it is known that when large-sized molded articles such as bumpers are formed of a material having poor moldability by means of injection molding, a tiger stripe-like molding appearance defect (flow mark) is generated, and such a defect results in a problem of dropping a commercial value.
That is, with respect to the performance of propylene-ethylene block copolymers enumerated in the foregoing conventional technologies, though an enhancement of the impact resistance is observed to some extent, not only there is room for an improvement of the heat resistance as yet, but also an effect for improving the moldability is not substantially recognized. For those reasons, in propylene block copolymers obtained by the conventional methods, there were encountered problems of a more still stiffness-impact resistance balance and improvements of the heat resistance and moldability, especially an appearance such as flow mark.